Pentagon Concerned About Beijing's Expanded Submarine Forces

SHANGHAI—A close midair encounter between U.S. and Chinese military planes last week reflected long-running tensions in the skies—a rivalry that is building under the waters below, as well.

The Defense Department didn't explain the mission for the U.S. Navy P-8 aircraft—a plane designed to track submarines—that was intercepted by a Chinese J-11 fighter over the South China Sea to the east of China's Hainan Island on Aug. 19. U.S....